Tin plate having a tin-nickel-iron alloy layer and method of making the same



IRON ALLOY LAYER KING THE SAME 19, 1962 TIN PLATE HA AND METH Fil G. G.MM ETAL vlNG A T -NICKEL- OD OF MA ed Nov.

lllllllll A Tra/QA/fys United States Patent O TIN PLATE HAVING ATiN-NICKEL-lRON ALLOY LAYER AND METHOD F MAKING THE SAME Gilbert GeorgeKamm, Barrington, and Allen Ray Willey,

Mundelein, Ill., assignors to American Can Company,

New York, NY., a corporation of New .Hersey Filed Nov. 19, 1962, Ser.No. 238,698 13 Claims. (Cl. 29-196A) This invention relates to coatedferrous metal sheet yand the method of making the same. Moreparticularly, the invention pertains to ferrous metal sheet covered witha composite coating comprising an outer layer of tin and an intermediatelayer of a tin-nickel-iron lalloy between the ferrous metal sheet andthe tin outer coating, and to the method of making lthe same.

Although the conventional tin can, which is essentially made of sheetsteel covered with a thin layer of metallic tin, has proved veryeffective as a container for corrosive products, there have beencontinuing efforts over the years to improve the corrosion resistance ofthe cans in order to increase the length of time that corrosive productsmay be sealed within the containers without spoilage of the product.

With the advent of the electrotinning process for coating steel sheet,rather than the hot dip method, the thickness of the tin deposit hasdecreased significantly. This in turn has increased the susceptibility-of the material to corrosion by the products within cans made from thesheet.

Based upon various tests, tin plate having superior corrosion resistanceis designated grade A plate, whereas tin plate having average corrosionresistance is designated grade B plate. It follows that a large numberof products packed in cans made from grade A plate have a longer shelfor storage life than in cans made tfrorn grade B plate. The presentinvention is especially concerned wit-h the simpler production of platewhich consistently has corrosion resistance equal to 4or better thancommercial .grade A plate.

It is therefore an object of this invention to provide a coated steelthat will have superior corrosion resistance .and a method for makingsuch a material.

Another object is to provide a composite coated ferrous metal sheet foruse in the manufacture of metal containers.

An additional object is to provide a ferrous metal sheet having thereonuniformly thin coating layers which increase the corrosion resistance ofthe sheet.

A further object of this invention is to provide tin plate havingcorrosion resistance equal to or -better than present-day commercialgrade A plate even though the tin is deposited from an acid tin platingbath.

A still further object is to provide a corrosion-resistant tin platewhich gives cans made therefrom a longer storage or shelf life whenpacked with products which corrode the can.

Yet another object is to provide a relatively simple and -less expensivemethod of making tin plate having irnproved corrosion resistance whichmay be readily incorporated into present-day, commercial tin plateproduction.

Numerous other objects and advantages of the invention will be apparent`as it is better understood from the following descrip-tion, which,taken in connection with the accompanying drawings, disclose a preferredembodiment thereof.

The .above and other objects of `the present invention are obtained byproviding a ferrous basis metal sheet having thereon an outer coating offlow-brightened tin and a tin-nickel-iron alloy layer between the tinouter 3269,53@ Patented July l2, i966 ice coating and the Aferrous basismetal. rI'his ternary alloy is obtained by the co-deposition of tin andnickel over the basis metal and thereafter depositing a layer of t-inover the intermediate tin-nickel layer. Subsequently the coated sheet isheated to a temperature sufficient -to melt the deposited tin coatingand alloy a portion of the basis ferrous metal land tin with thevdeposited binary alloy coating, thus forming a composite sheet having abright tin outer coating and a ternary Sn-Ni-Fe alloy intermediatealloy.

Referring to the drawings:

FIG. l is a perspective view of a container made from the material ofIthis invention.

FIG. 2 is an enlarged fragmentary sectional view taken substantiallyalong line 2-2 of FIG. 1.

As a preferred or exa-mplary embodiment of the present invention, FIG. lillustrates a metal container, generally designated 20, 'having a body22, and upper and lower ends thereon 23, 24 respectively, made integralwith the body 22 by any suitable means such as upper and lower doubleseams 25, 26 respectively.

The material, from which the container is made, is a composite coatedmetal sheet having a tin-nickel-iron ternary alloy 32, lying between anouter bright tin coating 34 and a ylow carbon ferrous basis metal 36(FIG. 2).

it may sometimes be desirable to coat the inner surface of the container20 with a synthetic resin 38. This would, of course, depend upon theproduct to be packaged. Among many resins that may be utilized `for thispurpose are phenolics, epoxies, crganosols, vinyls, etc.

The tin-nickel-iron ternary alloy 32 is a thin, substantially continuouslayer formed by diffusing a tin-nickel intermediate layer with the basismetal 36 and an asdeposited or .matte tin coating, by a method whichwill be described more `fully hereinafter.

Thickness of the ternary yalloy 32 will range from 0.25 X10*6 to 8X106inch, with a thickness of 1 10r6 to i6 l0t6 inch preferred. Although thethickness of the alloy is very small, it is thought that its uniformityand excellent coverage of the basis metal l3,6, together with itsmetallurgical composition, results in increased ,corrosion resistance ofthe composite sheet when cornpared to conventional tin plate.

Tin plate made according to conventional procedures generally shows thatneither the bright outer tin 'layer nor the tin-iron alloy layer lyingbetween the tin and the steel basis metal are completely continuous. Inother words, voids or discontinuities exist in each of these layers.Some of the voids in each layer are in alignment, so that through thembasis steel is exposed to the environment in contact with the tin plate.Obviously, the more discontinuous either layer is, the greater theincidence of alignment and consequently the greater the amount of basissteel exposed. These exposed `areas of base steel are, in some products,vulnerable to direct corrosive attack, while in others they acceleratecorrosion of tin from the plate surface.

In the present invention the Sn-Ni-Fe alloy layer is substantiallycontin-uous over the ferrous metal surface, thus obviating thediscontinuity usually encountered with the Sn-Fe alloy layer inconventional tin plate. It is thought that this continuity is primarilydue to the presence of nickel in the alloy, which acts to promoteuniform nucleation during flow brightening of the as-deposited tincoating, which process will be described in more detail hereinafter.

The composite metal of this invention is produced by providing lowcarbon ferrous metal sheet, preferably in coil form, which iselectrolytically cleaned in a hot alkaline cleaning solution, by meanswell known to those skilled in the art. Thereafter the strip is pickledin a 5% 3 sulfuric acid solution to remove any oxides present on themetal surface.

Once the surface contamination has been removed by cleaning andpickling, the strip is rinsed and then immersed in a tin-nickel platingbath having the fol-lowing nominal composition:

Ammonium hydroxide to pH 2.5.

Using the above tin-nickel alloy plating solution, a steel sheet,cleaned and pickled according to the method hereinbefore describe-d, wasimmersed in the electrolyte, Whose temperature was maintained at 150 F.A direct current at a density of 25 amperes per square foot was thenpassed through the electrolyte for 2 seconds, while Iagitation of thesolution was kept at a minimum. A total current of 50 ampere-seconds persquare foot was thus passed, -resulting in a tin-nickel alloy deposithaving a thickness of approximately 1.1 -6 inch.

Alloy anodes -may be used in the plating operation. However, the usualproced-ure, which is what was used in the example, is to employ separateanodes of tin and nickel, using one tin anode for each two nickelanodes. An alternate method is to use `only nickel anodes and tomaintain the tin concentration in the bath through the periodic additionof anhydrous stannous chloride. Deposition carried out at a currentdensity of from amperes per square foot (asf.) to 600 asf., dependingupon the degree of solution agitation, at a temperature of approximately150 F.

During the coplating operation, the current is passed for a time of0.025 to 10 seconds, With an optimum plating time of 1 to 4 seconds and2 seconds preferred. It is readily apparent that the plating time willvary according to the current density employed. This Will give a deposithaving a nominal composition of 65% tin and nickel. This composition hasthe nominal atomic percentage of Sn-Ni, with the deposited compositionremaining substantially constant over a Wide range of operatingconditions and solution variables.

The Sn-Ni thickness will range from 0.275 10-s inch to 5.5 106 inch,With a preferred deposit thickness of 1.l 106 inch. The thickness rangemay be produced by passing from 6.25 to 250 ampere-seconds of currentper sq. ft.

Although electroplating of the alloy coating is preferred, primarily foreconomic reasons, other techniques such as vacuum deposition or gaseousdeposition may be used.

Subsequent to the deposition of Sn-Ni the strip is rinsed and thenimmersed in a tin plating bath. Either an alkaline or acid tin platingbath may be used, but the acid type is considered more desirable due tothe higher current densities that may be used, and consequently theshorter plating cycles that may be attained.

One of numerous acid tin plating baths that may be used has acomposition as follows:

Oz. per gal. Stannous sulfate 13 Sulfuric acid 4 Tartaric acid 4 Animalglue 0.4-0.8 Cresol or 0.8 B-napthol 0.13

The bath is operated at room temperature and pure tin `anodes are usedin order to minimize the formation of sludge during plating. i

Using a current density of from 10 to 400 a.s.f., dependent upon thedegree of agitation in the bath, a coating of tin from 10X10*6 to 90x106inch in thickness is deposited upon the intermediate Sn-Ni layer.Preferably the tin deposit will have a thickness from 30 10 6 to 60x10*6inch.

It is possible to deposit the tin coating by such techniques as vacuumor gaseous deposition, although the hereinbefore mentioned electrolyticmethod is preferred.

Once the composite coating has fbeen deposited upon the ferrous -metalstrip, the next operation in the process is to flow brighten thelcoating and thereby produce a bright tin surface, in addition totransforming the intermediate Sn-Ni alloy coating to a Sn-Ni-Fe ternaryalloy.

Either resistance or induction heating may `be used to heat the coatedstrip. By means of either heating method the coated metal is heated to atemperature ranging from 450 F., the melting point of tin, toapproximately 550 F. The temperature is maintained above 450 F. for aperiod of 0.01 to 1 second, and preferably 0.25 second.

The flow brightening step causes the tin to flow out into a smooth,bright film and to ybecome rmly anchored to the alloy layer. Inaddition, the heating during flow brightening results in the formationof the tin-nickel-iron ternary alloy which lies between the bright tinouter coating and the ferrous 'basis metal.

Although not wishing to be `bound by any particular theory as to thereaction in forming the ternary alloy layer during flow brightening, itis thought that some of the melted tin diffuses into the tin-nickelalloy and then some surface iron in contact with the tin-nickel alloycombines with the binary alloy in a second diffusion reaction. Thediffusion reactions may proceed simultaneously. During the diffusionprocess the nickel, in the now ternary alloy, functions as centers fornucleation, thus resulting in uniform nucleation and a uniform alloylayer.

Although satisfactory results are obtainable by flow brightening the tinat temperatures substantially above its melting point, for best results,both as to appearance and corrosion resistance, the tin should -befiowed at temperatures above, but as close as possible to, the meltingpoint of tin. It is preferred that the tin remain at the temperatureabove its melting point for as short a time as possible. To this end,the molten tin is solidified as rapidly as possible, such as by a coolair bath or a water quench or both.

One test which is used in determining the quality of tin plate is thealloy-tin-couple test or, as it is most cornmonly called, the ATC test.Attention is given to the electrochemical factors important to tin platecorrosion in this test in which steel is protected from corrosion bycoupling with tin. The rate of detinning or corrosion is determined -bythe galvanic couple current and is measured in microamperes per squarecentimeter. ATC measurements for typical electrolytic tin plate willgenerally range from 0.02 to 0.30 microampere per square centimeter,with the grade A plate having the lower values and grade B plate,usually resulting from commercial acid tin plating procedures, havingthe higher values. When tin plate made according to the presentinvention, even plate on which the tin was deposited from an acidplating bath, was subjected to the ATC test, the values ranged from only0.015 to 0.050 microampere per square centimeter. Thus, with plate madeaccording to this invention, -it is possible to obtain ATC values evenlower than for conventional grade A tin plate.

It is thought that the invention and many of its attendant advantageswill be understood from the foregoing description, and it will beapparent that various changes may be made in the form, construction andarrangement of the parts and that changes may -be made in the steps ofthe method described and the order of accomplishment without departingfrom the spirit and scope of the invention or sacrificing all of itsmaterial advantages, the form hereinbe'fore described being merely apreferred embodiment thereof.

We claim:

1. A coated ferrous metal sheet, comprising: a ferrous basis metal; aninner coating over and in contact with said basis metal consistingessentially of a ternary alloy of tin, nickel, and iron; and an outercoating of tin over and in contact with said ternary alloy, said ternaryalloy being produced by the diiusion of an intermediate deposit of abinary tin-nickel alloy with said ferrous basis metal and said outer tincoating while said tin coating is molten.

2. The sheet of claim 1 wherein said ternary alloy coating has athickness from 0.25)(-6 to 8X106 inch.

3. The sheet of claim 2 wherein said ternary alloy coating has athickness from 1x10"6 to 6x10*6 inch.

4. The sheet of clairn 2 wherein said outer tin coating has a thicknessfrom 10X106 to 90x10-6 inch.

5. A method of manufacturing a composite coated ferrous metal sheet,comprising the steps of: depositing a layer of a tin-nickel binary alloyon a ferrous basis metal sheet; maintaining said binary alloysubstantially in its as deposited condition while depositing a layer oftin over said -binary alloy, said tin deposit having a thickness greaterthan said deposit of tin-nickel alloy; heating said coated ferrous sheetto a temperature above `450 F., for a time sufficient to melt said tincoating and to diffuse iron from said basis metal into throughout saidbinary alloy while said -basis metal and said binary alloy are solid toform a tin-nickel-iron ternary alloy over and in contact with said basismetal; and thereafter cooling said coated sheet to a temperature belowthe melting point of tin to solidify said tin as a surface coating overand in contact with said ternary alloy.

6. The method of claim 5 wherein said tin-nickel alloy iselectrodeposited.

7. The method of claim 5 wherein said tin-nickel alloy is deposited to athickness from 0.275 X10s to 5.5 10*s inch.

8. The method of claim 5 wherein said tin is electrodeposited.

9. The method of claim 5 wherein said tin is deposited to a thicknessfrom 10X10*6 to 90 106 inch.

10. The method of claim 5 wherein said ternary alloy has a thicknessfrom 0.25 X10*G to 8X106 inch.

11. The method of claim 5 wherein said bright tin has a thickness from10X106 to 90 10d6 inch.

12. The method set forth in claim 5 wherein said coated ferrous sheet ismaintained at said temperature above 450 F. for a period of from 0.01 to1 second.

13. The method set forth in claim 12 wherein said coated ferrous sheetis maintained at a temperature 'between 450 F. and 550 F.

References Cited by the Examiner UNITED STATES PATENTS 2,085,543 6/1937opiinger 2044-40 2,266,330 12/1941 Nachiman 204-375 2,428,318 9/1947Naehtman 204-375 2,775,535 12/1956 Poole 204-37.5 2,914,449 11/1959Mayer 204-43 3,062,726 11/1962 Hin 204-375 WINSTON A. DOUGLAS, PrimaryExaminer. MURRAY TILLMAN, Examiner. L. G. WISE, H. S. WILLIAMS,Assistant Examiners.

1. A COATED FERROUS METAL SHEET, COMPRISING: A FERROUS BASIC METAL, ANINNER COATING OVER AND IN CONTACT WITH SAID BASIS METAL CONSISTINGESSENTIALLY OF A TERNARY ALLOY OF TIN, NICKEL, AND IRON, AND AN OUTERCOATING OF TIN OVER AND IN CONTACT WITH SAID TENARY ALLOY, SAID TENARYALLOY BEING PRODUCED BY THE DIFFUSION OF AN INTERMEDIATE DEPOSIT OF ABINARY TIN-NICKEL ALLOY WITH SAID FERROUS BASIS METAL AND SAID OUTER TINCOATING WHILE SAID TIN COATING IS MOLTEN.